1. Field of the Invention
The invention relates to a method for producing an arc-erosion resistant coating, in particular for inner regions of shields that are exposed to electric arcs, and to a shield produced in this way for vacuum interrupter chambers.
In particular in inner regions of switching devices that are exposed to electric arcs, for example so-called vacuum interrupter chambers, arranged inside which are contact pieces which pull apart the closed switching contacts in a permanent vacuum by means of a mechanism acting from the outside, possibly on the basis of an appropriate displacement-time curve, internal components undergo strong thermal and plasma-related stress.
2. Description of Related Art
Such vacuum interrupter chambers are used in low-voltage, medium-voltage and high-voltage switchgear. Electric arcs that are created between contact pieces in a vacuum atmosphere, in particular under short-circuit current conditions during switching off (separation of the contact pieces), are extinguished the next time the current passes through zero, or at the latest the next-but-one time it passes through zero. However, they act on the inner regions of the vacuum interrupter chamber only for milliseconds and, as is known, high energy densities thereby occur, even if only for a brief time. This means that there is quite a significant load on at least some of the components of a vacuum interrupter chamber in the case of a compact design of a vacuum chamber, and so the service life of such a vacuum chamber is substantially limited by the number of switching operations that are performed in the case of a short-circuit.
For this reason, some of the vacuum interrupter chambers are equipped with an arc-erosion resistant shield, which is positioned between the surrounding area of the contact pieces and the inner wall of the interrupter chamber (for example the ceramic insulator).
Since the shields are thin-walled, cylindrical, partly contoured, sheet-metal parts, their plasma erosion is particularly high under the heat correspondingly produced.
Furthermore, sintering methods for producing copper-chromium shields by means of a powder sintering process are known from the prior art. For this purpose, pressing tools for producing the green compacts are required for the different diameters. The production of a compact material subsequently takes place by sintering the green compacts at temperatures of around 1000 degrees Celsius under a vacuum or an inert-gas atmosphere.
Furthermore, the plasma spraying method is known as an example of a thermal spraying method. The thermal method is used for applying a copper-chromium layer. Plasma spraying is carried out in a known way on the basis of the strong getter effect of the chromium in an inert-gas atmosphere. An increased gas content in the sprayed-on layer is unavoidable however, and is disadvantageous.
Furthermore, so-called MLC methods are known, used for producing a sheet form for vacuum interrupter chamber shields or vacuum interrupter chambers, according to DE 19747242 C2.
The use of a copper-chromium shield provides a broader scope for the structural design of a compact vacuum interrupter chamber. However, this entails higher costs of the vacuum interrupter chamber. This disadvantage can be partly reduced by integration of the shield and arc-erosion resistant layer. Nevertheless, the arc-erosion resistance is limited and at the same time the methods referred to are relatively complex. Moreover, certain material compositions, that is to say the variation of the stoichiometric element, are also much more difficult here.